Time domain reflectometry (TDR) has been used to measure the volumetric moisture content of soils (volume of moisture per unit volume of soil), mostly in the field of soil science. As shown in FIG. 1, these measurements involved the insertion of a probe 10 comprising a central rod 12 and two or more peripheral rods 14 into the soil 16 to be measured. The peripheral rods 14 (which are preferably three in number) are spaced equidistant from the central rod 12 and equidistant from each other. A coaxial transmission line 18 is then coupled to the structure with the center conductor of the coaxial cable 18 coupled to the center rod 12 and the exterior shield (outer conductor) of the coaxial cable 18 coupled to each of the peripheral rods 14. In this way, the peripheral rods 14 simulate the effects of a continuous outer coaxial shield in the soil 16, without the requirement of attempting to drive a cylindrical probe into the soil 16. Time domain reflectometry analysis equipment 20 is then coupled to the coaxial cable 18, and the reflections of high frequency electrical signals from the soil 16 are measured using the analysis equipment 20. These reflections will change in predictable ways depending upon the dielectric constant of the soil 16, which has been found to be strongly correlated with the volumetric moisture content of the soil 16. Therefore, time domain reflectometry has been established as a viable tool for measuring volumetric moisture content of a soil.
An innovative improvement was made by Siddiqui and Drnevich, U.S. Pat. Nos. 5,801,537; 5,933,015; and 6,215,317, which are hereby incorporated by reference, to extend TDR to geotechnical applications.1 They developed a calibration equation relating soil apparent dielectric constant to soil gravimetric water content and dry density and designed procedures for laboratory calibration and field application. The laboratory calibration was done in conjunction with a standard compaction test. The field procedure consisted of two tests: 1) a test in which a TDR reading was taken on a plurality of spikes driven into the soil; and 2) a test in which a TDR reading was taken in a compaction mold on the same soil that was rapidly excavated from within the volume bounded by the spikes. The spikes formed a coaxial probe for the first test and a single rod driven into the center of the soil in the compaction mold formed a coaxial mold probe for the second test. Assuming that the water content was the same for both tests, the apparent dielectric constant from the two TDR readings and the measured total density of the soil in the mold were used to calculate soil water content and dry density. Laboratory and field evaluations indicated that the method had sufficient accuracy for geotechnical purposes.2,3,4,5 An ASTM standard designated ASTM D6780 for the method was recently approved. The procedure described above made use of measured apparent dielectric constants (one with soil in place and one with soil in the mold). It also required digging out the soil and compacting it into a mold. This process required about 10 to 15 minutes. 1 Siddiqui, S. I. and Drnevich, V. P. (1995). “A New Method of Measuring Density and Moisture Content of Soil Using the Technique of Time Domain Reflectometry,” Report No.: FHWA/IN/JTRP-95/9, Joint Transportation Research Program, Indiana Department of Transportation—Purdue University, February, 271 p.2 Lin, C. P. (1999), “Time domain reflectometry for soil properties”, Ph.D. Thesis, School of Civil Engineering, Purdue University, West Lafayette, Ind.3 Siddiqui, S. I., Drnevich, V. P. and Deschamps, R. J. (2000). “Time Domain Reflectometry Development for Use in Geotechnical Engineering,” Geotechnical Testing Journal, GTJODJ, Vol. 23, No. 1, March, pp. 9–20.4 Drnevich, V. P., Lin, C. P., Yi, Q., Yu, X. and Lovell J. (2001b), “Real-time determination of soil type, water content and density using electromagnetics”, Report No.:FHWA/IN/JTRP-2000-20, Joint Transportation Research Program, Indiana Department of Transportation—Purdue University, August, 288 p.5 Drnevich, V. P., Yu, X., and Lovell, J., 2002, A New Method for Water Content and Insitu Density Determination, Proceedings of the Great Lakes Geotechnical and Geoenvironmental Conference, Toledo, Ohio, May, 15p
A multiple rod probe (MRP) 22 of the prior art according to Siddiqui and Drnevich is illustrated in FIGS. 2–4. The MRP 22 was used to measure the dielectric constant (and hence the volumetric moisture content) of an in-place soil sample. The conducting rods 24 of the MRP 22 were driven into the soil 26 in a predetermined pattern using a guide template 28 placed upon the surface of the soil. The pattern included a centrally located rod and two or more peripherally located rods, all being equidistant from the central rod. The rods 24 were preferably common metal spikes, and extended into the soil to a depth of approximately nine inches. The template 28 was removed after the rods 24 were driven into the soil. The MRP 22 further included an interface cap 30 which was formed from a conductive material, such as stainless steel. The cap 30 had a plurality of studs 32 extending downwardly therefrom. The centrally located stud was electrically insulated from the interface cap 30, while the peripheral studs were mounted in electrical contact with the conductive portion of the cap 30. A coaxial connector 34 was mounted to the cap 30 such that the outer conductor was in electrical contact with the conductive portion of the cap 30 and the peripherally located studs 32. The center conductor of the connector 34 was in electrical contact with the centrally located stud but was insulated from the conductive portion of the cap 30. The connector 34 was coupled to a TDR instrument 20 by means of a coaxial cable 18.
It would be a desirable improvement to the above-described prior method and apparatus to make use of only the multiple rod probe, and eliminate the necessity of excavating and compacting soil into a compaction mold in the field. The present invention provides this and other desirable improvements.